The present invention relates to creating highly uniform magnetic fields in magnets used for magnetic resonance imaging, by the use of passive shimming to improve magnetic field homogeneity.
To create a highly uniform magnetic field with an electromagnet or array of permanent magnets, it is necessary to build the magnets to a carefully specified shape, and to strive to minimize the deviations from the specified shape due to manufacturing variations. The resulting magnets, however, typically require field corrections to achieve the desired level of inhomogeneity, due to deviations of the magnet from the designs or due to the presence of ferromagnetic material in the vicinity of the magnet.
To improve field uniformity, correction coils are typically used. These coils are capable of creating different field shapes which can be superimposed on an inhomogeneous main magnetic field to perturb the main magnetic field in a manner which increases the overall field uniformity. Unfortunately, many sets of such coil are typically required. A state of the art magnetic resonance (MR) imaging magnet has between 10 and 20 independent sets of corrections coils, each with its own power supply to provide the correct current flow. Naturally, these coils add significantly to the cost and complexity of the magnet.
Passive shimming is currently used to correct large deviations in magnetic fields that cannot be corrected by the available correction coils alone. The passive shimming is accomplished by placing a piece of iron in an appropriate place outside the magnet. The desired level of field uniformity can then be achieved by the correction coils.
A passive shimming assembly must provide accurate and reproducible placement of the passive shims in the magnet to avoid difficulties in removing and returning shim carriers to exactly the same location, the assembly must be rigid and accurately referenced to the magnet cryostat. The radial thickness should have a small radial extent to take up as little as possible of the magnet bore space thereby leaving room for the radio frequency coils and patient table.
It is an object of the present invention to provide a passive shimming assembly for a magnetic resonance magnet which can achieve a level of field inhomogeneity required for magnetic resonance imaging, without the use of correction coils.
It is a further object of the present invention to provide a shimming assembly which when used with a superconducting magnet can be adjusted without de-energizing the magnet.
It is another object of the present invention to provide a rigid assembly for mounting and locating the shim carriers while reducing the radial intrusion of the assembly into the magnet bore.